Solders are generally used for bonding metal materials, and are supposedly alloy materials having a melting temperature range (a range from a solidus temperature to a liquidus temperature) of 450° C. or less. Conventionally, when electronic components are mounted on printed boards, an Sn-37Pb eutectic solder of 183° C. in melting point is used and the reflow heat treatment is performed mainly in the temperature range of about 200° C. to 230° C. The reflow heat treatment condition is generally set at a temperature range of 10 to 50° C. higher than the melting point of a solder alloy.
However, recently, as in the environmental regulations of the EU (RoHS directive), the problem with the hazardousness of Pb arises and making solders lead-free rapidly progresses in view of preventing contamination of the environment and human bodies. In such a situation, it is now becoming general that as a substitute for the Sn-37Pb eutectic solder, a lead-free solder composed of Sn-3.0Ag-0.5Cu having a melting point of about 220° C. (see Patent Document 1) is used and the reflow heat treatment is performed at a temperature range of about 240° C. to 260° C.
Meanwhile, since the above-mentioned lead-free solder, having a melting point of about 220° C. and mainly composed of Sn has a higher melting point than that of the Sn-37Pb eutectic solder, the reflow heat treatment condition necessary during use must be naturally a higher temperature. Recently, for suppressing thermal damage to electric and electronic devices, soldering at a temperature as low as possible is demanded and bonding materials which are not only lead-free but also have a heat resistive performance at the conventional reflow heat treatment condition corresponding to the Sn-37Pb eutectic solder have been studied.
As components reducing the melting points of lead-free solder alloys, Bi, In, Zn and the like have been confirmed to be effective, but the melting point reduction is sometimes insufficient depending on the amount ratios of them. Bi improves wettability of a solder alloy to a substrate, but is liable to segregate on solidifying, bringing about a brittle crystal texture and a poor ductility. Therefore, when it is added to the solder alloy at a certain amount or more, the mechanical strength is outstanding reduced (see Patent Documents 2 and 3). It is a rare resource and a very expensive material. Therefore, its addition at a large amount to a solder alloy results in a large cost increase (see Patent Documents 4 and 5). Zn is inexpensive and has favorable mechanical properties, and then is expected to be put in practical use. However, Zn has a very high activity and properties of being easily reacted and easily oxidized. Therefore, a solder alloy paste containing Zn exhibits a poor paste stability and a low corrosion resistance. Further, bonding of Zn with Cu forms not a Cu—Sn intermetallic compound layer but a Cu—Zn intermetallic compound layer on the interface. The Cu—Zn intermetallic compound layer has such problems as remarkable strength deterioration under a high temperature and high humidity environment (see Patent Document 6).
The present inventors previously has proposed lead-free conductive materials capable of connecting at a lower heat treatment temperature than that of the Sn-37Pb eutectic solder (see Patent Documents 7, 8, 9 and 10). However, since these conductive materials have a feature in exhibiting a raised lowest melting point after the connection by the heat treatment and developing the connection stability, they have neither a melting point which does not change before and after the connection nor repairability, as is the case with common solder materials.    Patent Document 1: JP-A-05-050286    Patent Document 2: JP-A-05-228685    Patent Document 3: JP-A-08-206874    Patent Document 4: JP-A-08-187591    Patent Document 5: International publication WO 2006/080247 pamphlet    Patent Document 6: JP-A-06-238479    Patent Document 7: JP-A-2004-223559    Patent Document 8: JP-A-2004-363052    Patent Document 9: JP-A-2005-005054    Patent Document 10: International publication WO 2006/109573 pamphlet